1. Field of Invention
The present invention relates to a digital control unit, and more particularly relates to a unit to control an idling engine speed in such a manner that: consideration is given to the internal state of a system to control the idling speed of an internal combustion engine; the system is grasped as a dynamic system; and the idling engine speed is controlled corresponding to a control value which is found in accordance with a state variable which determines the internal state.
2. Description of the Related Art
Conventionally, this kind of idling speed control unit of an internal combustion engine has been disclosed, for example, in the official gazette of Japanese Patent Application Laid Open No. 145339-1984. In this official gazette is shown a composition characterized in that: when the engine condition is changed from the one in which the idling engine speed is not fed back, to the one in which the idling engine speed is fed back, the engine speed can be smoothly changed to a target value.
However, in the composition shown in the aforementioned official gazette, when it has been judged that the feedback control of the engine speed is started, the initial values of the state variable used when the control value to control the engine speed is found, and the initial value of the integral term of the deviation between the detected engine speed and the target engine speed, are given in accordance with the engine speed detected when the throttle valve has been completely opened, and the engine speed detected when it has been judged that feedback control is started. Therefore, the initial value of the state variable and that of the integral term must be prepared corresponding to the engine speed detected when the throttle valve has been completely closed, and the engine speed detected when it has been judged that feedback control is started. Even when the initial values are finely set corresponding to the aforementioned engine speed, as the internal state of the system to control the idling speed of an internal combustion engine and the engine speed can not be determined definitely, it is not clear whether the given initial values are appropriate or not. Therefore, there is a possibility that the convergence to the target engine speed is deteriorated depending on a case.
Accordingly, the present invention has been achieved to solve the aforementioned problems. It is a primary object of the present invention to provide a digital control unit characterized in that: the convergence to a target engine speed can be improved when the engine condition is changed from the one in which the aforementioned actuator is driven without any relation with the internal state of an internal combustion engine, to the one in which the aforementioned actuator is driven by the value which is set according to the internal state of the internal combustion engine.
When a disturbance load such as a load of an air-conditioner is given to an engine, the transient response of the idling engine speed is conventionally improved by correcting a value computed by feedback control in accordance with the disturbance.
In the case of an idling speed control unit of an internal combustion engine, consideration is given to the operable range of an actuator (for example, an idling speed control valve) to control the idling engine speed, and the finally outputted control value (the duty ratio of a duty signal to control a control solenoid) is generally controlled so that it can be within a predetermined upper and lower limit (0-100%). As shown in the official gazette of Japanese Patent Application Laid Open No. 43942-1984, the upper and lower limit are also set to the integral value and the state variable, in order to prevent the actuator from being affected when the control value found by the previous calculation timing used for determining the finally outputted control value, the state variable determined by the detected engine speed, and the integral value of the deviation between the target engine speed and the detected engine speed, become excessively high or low.
Therefore, when the conventional composition in which the calculation value obtained by feedback control is corrected by the correction in accordance with a disturbance, is applied to the aforementioned apparatus in which the upper and lower limit are set to the final control value, the integral value, and the state variable, the following problems are caused: even when the detected engine speed is higher than the target engine speed and the final control value is made minimum, the correction is added as a load disturbance is given, so that the engine speed can not be reduced to not more than the value which is larger than the lower limit of the final control value by a predetermined value. In this case, the idle speed control valve is maintained in a condition in which it is opened by a predetermined open value. Therefore, the detected engine speed can not be converged to the target engine speed, so that there is a possibility that the engine speed is maintained high.
Consequently, the second object of the present invention is to provide an improved digital control unit which is characterized in that: when the control value is corrected in the manner of feedforward, giving consideration to the influence caused by a load disturbance, the detected engine speed can be very precisely controlled to the target engine speed, without causing the aforementioned problems.
The third object of the present invention is to provide a digital control unit characterized in that: a highly precise dynamic model of low degree is constructed, and very accurate control can be conducted according to the constructed dynamic model.